This invention relates to a vehicle drive system for vehicles, such as for an agricultural or industrial tractor, wherein the drive system includes a drive axle with two wheels, each wheel being driven by a respective electric motor powered by an engine-driven generator.
Such electric individual wheel drive systems are known from DE-A 4 108 647 and U.S Pat. No. 4,953,646. Such drive systems permit individual matching of the drive power to the different driving requirements. Thus, stepless adjustment of the wheel speed in the forward and reverse directions is possible using a suitable controller for the electric motors. Furthermore, driving the wheels at different speeds and individually matched torques is advantageous when travelling around a curve, since it is possible to avoid deformation of the vehicle, to reduce tire wear and attain improved tracking. However, because of the conversion of the whole of the mechanical energy into electrical energy and back into mechanical energy, the efficiency of the known drive systems is not satisfactory for all applications.
Vehicle drives are also known (B. R. Hohn & B. Pinnekamp: "Der autarke Hybrid--Ein universelles Antriebskonzept fur Pkw", ATZ Automobtechtlische Zeitschrift 96(1994), page 294), in which the wheels of an axle are driven in common. In a first drive concept (serial hybrid), the drive to the axle is always provided by a single electric motor, which is powered by an engine driven and generator unit or alternatively by a vehicle battery. There is no purely mechanical coupling between the engine and the axle. According to a second drive concept (parallel hybrid), the axle is driven alternatively by the engine (and a mechanical transmission) or the electric motor, which draws its energy from the vehicle battery. Such drive systems can suit private cars but they do not solve the problems which occur with commercial vehicles relating to efficiency, driving behavior and provision of the required energy.